In order to print an image with a printer, it is typically necessary to perform a quantization process on the image. The quantization process is a halftone process of converting an image expressed with a continuous tone into a gradation which can be expressed by the printer. For example, a dither process or an error diffusion process has been known as a method of performing the quantization process.
A method of performing the quantization process selectively using the dither process and the error diffusion process has been known (for example, see Japan Laid Open Publications JP-A-2006-240054 and JP-A-2008-87382). For example, in the method disclosed in Japan Laid Open Publications JP-A-2006-240054, the dither process and the error diffusion process are separately used depending on the gray level ranges. In the method disclosed in Japan Laid Open Publications JP-A-2008-87382, the dither process and the error diffusion process are separately used depending on dot sizes by the use of a print head which can form N types of dots having different sizes.